At present, the use of inorganic microparticles in the retention system of paper production, in particular in the production of fine paper, is very common, the aim being to improve further the efficiency of the production process. The advantages of the introduction of microparticles include improved retention, more efficient dewatering, and better formation. The most effective of the microparticles in use are colloidal silica-based microparticles of various types, solid or sol, and bentonite-like swellable natural materials belonging to the smectite group of clays. Instead of, or in addition to, a microparticulate compound it is possible to use as a retention aid in the retention system polymers, which may be anionic, cationic or non-ionic, and which are characterized by a high molecular weight. The problem involved with these compounds is typically excessive flocculation, which deteriorates the optical properties of paper.
Bentonite has been used as a retention aid in paper production together with a cationic polymer in the U.S. Pat. No. 4,753,710. In the process according to this patent, a cationic polymer, preferably polyethylene imine, a polyamine epichlorohydrin product, a polymer of diallyl dimethyl ammonium chloride, or a polymer of acrylic monomers, was added to an aqueous cellulosic suspension before the last shearing stage, and bentonite was added after this shearing stage. Improved retention, dewatering, drying, and web forming properties were thereby achieved. In the microparticle system according to this process there is used bentonite, which is available under the trade name HYDROCOL.
The use of silicate microparticles together with a cationic polymer in a retention system is described in the U.S. Pat. No. 5,194,120. The prevalent cation in the synthetic amorphous metal silicate was Mg, and the polymer was preferably a ternary or quaternary amine derivative of polyacrylamide, their weight ratio being between 0.03:1 and 30:1. By this method, retention, dewatering and formation were improved by using smaller amounts of retention aids than previously, and thus the costs were correspondingly lower.
WO 01/40577 A1 discloses a method for the production of paper or board, wherein retention aids are added to the stream of stock. Improved retention and more effective dewatering are achieved by adding to the stream of stock a cationic polymer solution and a suspension-form microparticle mixture composed of a swellable clay of the smectite group, such as bentonite, and a colloidal synthetic metal silicate in which the prevalent cation is magnesium.
The most commonly used microparticles are inorganic materials, especially various minerals. Such minerals increase the ash content of the produced paper.
U.S. Pat. No. 4,483,743 discloses a process for manufacturing microfibrillated cellulose (MFC) by passing a liquid suspension of cellulose through a high pressure homogenizer having a small diameter orifice in which the suspension is subjected to a pressure drop of at least 3000 psig (20670 kPa) and a high velocity shearing action followed by a high velocity decelerating impact, and repeating the passage of said suspension through the orifice until the cellulose suspension becomes substantially stable. The produced MFC has a water retention value of over 280%. The MFC can be used with paper products and non-woven sheets to improve their strength. MFC produced by this type of process typically has a width of about 25-100 nm while the length is much longer.
U.S. Pat. No. 4,952,278 discloses a paper structure having both high opacity and improved tensile strength obtained by the incorporation of expanded cellulosic fibers and an opacifying mineral pigment, such as titanium dioxide. The expanded cellulosic fiber may be microfibrillated cellulose described in the above patent. The expanded cellulosic fibers are added in an amount of from 1% to 25%, preferably from 5% to 10% based on the dry weight of the opacified paper structure.
WO 2007/091942 A1 discloses an improved method for manufacturing microfibrillated cellulose. The disclosed method is said to solve the problems relating to clogging in high-pressure homogenizers and high energy consumption. According to this document microfibrillated cellulose is manufactured by refining a hemicelluloses containing pulp, preferably sulphite pulp, and treating the pulp with a wood degrading enzyme followed by homogenizing the pulp. The enzyme is a cellulase, preferably a cellulase of endoglucanase type which most preferably is a mono-component endoglucanase. The pulp can be refined before or after the enzyme treatment or both before and after the enzyme treatment. The obtained microfibrillated cellulose can be used in food products, cosmetic products, pharmaceutical products, paper products, composite materials, coatings or in rheology modifiers (e.g. drilling muds).
Yet another type of microfibrillated cellulose is described by Wågberg Lars et al., Langmuir 2008, Vol. 24, 2008, pages 784-795. This microfibrilled cellulose was prepared by high-pressure homogenization of carboxymethylated cellulose fibers. The fibers were sulfite softwood-dissolving pulp fibers. The produced MFC typically has a width of about 5-15 nm and a length which can be more than 1 μm.
Also other chemical pretreatment methods are known, such as an oxidation pretreatment of pulp fibers described by Saito et al. in Biomacromolecules, Vol. 8, No. 8, 2007, pp. 2485-2491. The pulp fibers are oxidized with a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated system followed by mechanical treatment. This oxidation pretreatment converts primary hydroxyl groups of the celluloses to carboxylate groups. The produced nanofibers typically have a width of about 3-4 nm and a length of a few μm.
One of the purposes of the present invention is to provide an organic substance which acts like a microparticle, which results in an improved retention as compared to mineral microparticles and which is made of a renewable material.